I guess another way of looking at it is, say I have two beers that have just finished fermenting. One I leave undisturbed and the other I use a wine whip and degas as much CO2 out of it that I can. If I let them both sit under the same amount of CO2 headspace pressure, do they eventually end up with the same amount of dissolved CO2, or does the undisturbed one hold more because it started from a supersaturated state?
I don't think Henry's law would ever get you back to a supersaturated state. Also, eventually may be very long if the beer sits undisturbed. CO2 will dissolve into the surface layer, but without mixing it kind of stops. At least, this is what happens in lakes - only a few different system variables there. But the idea is the same as temperature gradients around an immersion chiller.
A few years back I read a thread on another forum. The person had calculated the CO2 contribution from an ounce of priming sugar and showed that the priming sugar and dissolved CO2 along were not enough to provide the volumes of CO2 predicted by those charts. The rest must come from supersaturation and/or small amounts of continued fermentation.